1. Field of the Invention
This invention relates to a system for processing a surround signal. This invention relates to a method of processing a surround signal. This invention relates to an information recording medium which stores a surround signal processing program. This invention relates to a transmission system for a surround signal processing program. This invention relates to a reception system for a surround signal processing program. This invention relates to an apparatus for recording a surround signal. This invention relates to a method of recording a surround signal. This invention relates to a recording apparatus for a surround signal processing program.
2. Description of the Related Art
Systems for recovering multiple-channel audio signals are of several types such as the 3-1 type in High-Vision and the 4-channel matrix type based on Dolby surround. Many motion-picture films have surround tracks which carry surround information resulting from Dolby surround audio processing. In a motion-picture theater, sound information is reproduced from a surround track, and the reproduced sound information is decoded into multiple-channel sound signals. The sound signals are fed to loudspeakers before being converted into corresponding sounds, respectively. The loudspeakers include front loudspeakers and also a rear loudspeaker to provide the surround effect.
There are commercially available video tapes and laser discs which are made on the basis of such motion-picture films by steps including a step of copying sound information. These video tapes and laser discs store sound information which results from surround audio processing such as Dolby surround audio processing.
In a prior-art 4-channel surround audio system, since a rear sound signal (a surround signal) is monaural, it tends to be difficult to accurately express a feeling of certain types of forward or backward movement of a sound image. According to Dolby surround, since the 4-channel matrix is analog, perfect decoding tends to be difficult. In the prior-art 4-channel surround audio system, the frequency band of a surround track is considerably limited to prevent crosstalk.
Craig C. Todd et al in Dolby Laboratories have proposed a 5-channel audio format, "AC-3", which is also called "Dolby digital". According to the AC-3 format, rear sound is represented by discrete surround signals.
Specifically, according to the AC-3 format, a left signal, a right signal, a center signal, a left surround signal, and a right surround signal form a set of 5-channel signals which are multiplexed before being transmitted. Signals of the AC-3 format can be recorded on surround tracks in conventional package recording media.
During the reproduction of information from a conventional package recording medium having a surround track, sound information is reproduced from the surround track, and is decoded into 4-channel signals of Dolby surround or 5-channel signals of Dolby digital. It is known to convert such 4-channel signals or 5-channel signals into only 2-channel signals for an audio system without any rear loudspeaker. The 2-channel signals are fed to two front loudspeakers, respectively. The conversion of the 4-channel signals or the 5-channel signals into the 2-channel signals is designed to provide a virtual rear loudspeaker or virtual rear loudspeakers for the surround effect.
Japanese published unexamined patent application 8-51698 discloses a surround signal processor for 5-channel signals including a left surround signal and a right surround signal. The surround signal processor is followed by two loudspeakers. The surround signal processor converts the 5-channel signals into 2-channel signals which are fed to the two loudspeakers respectively. The surround signal processor includes a first adder, a second adder, a first subtracter, a second subtracter, a first filter, and a second filter. In the surround signal processor of Japanese application 8-51698, the first adder adds the left surround signal and the right surround signal, and outputs a signal representing the sum of the left surround signal and the right surround signal. The first subtracter subtracts the right surround signal from the left surround signal, and outputs a signal representing the difference between the left surround signal and the right surround signal. The first filter processes the output signal of the first adder. The second filter processes the output signal of the first subtracter. The second adder adds the output signal of the first filter and the output signal of the second filter, and outputs a signal representing the sum of the output signal of the first filter and the output signal of the second filter. The second subtracter subtracts the output signal of the second filter from the output signal of the first filter, and outputs a signal representing the difference between the output signal of the first filter and the output signal of the second filter. The output signal of the second adder and the output signal of the second subtracter are fed to the loudspeakers respectively. Transmission characteristics P of the first filter and transmission characteristics N of the second filter are set as P=(F+K)/(S+A) and N=(F-K)/(S-A), where "S" denotes transmission characteristics of the path from the couples of the loudspeakers to the ear of a listener on one side; "A" denotes transmission characteristics of the path from the couples of the loudspeakers to the ear on the opposite side; "F" denotes transmission characteristics of the path from a desired position of a localized rear sound image to the ear of the listener on one side; and "K" denotes transmission characteristics of the path from the desired position of the localized rear sound image to the ear on the opposite side.
A second surround signal processor disclosed in Japanese application 8-51698 serves to process 5-channel signals having a left surround signal and a rear surround signal. The surround signal processor is followed by two loudspeakers. The surround signal processor includes four sound image localizing filters. The left surround signal is transmitted to the two loudspeakers via two sound image localizing filters, respectively. The right surround signal is transmitted to the two loudspeakers via two sound image localizing filters, respectively.